The present invention relates to solar energy conversion apparatus and, more particularly, to improvements in solar energy collectors of the trough reflector type and to methods of manufacturing same.
Considerable time and effort is presently being expended in determining the feasibility of utilizing the sun's energy as a means for providing the heating and power needs of home and industry. While know-how exists for converting solar energy into vast quantities of heat and electrical energy, the principal stumbling block confronting widespread commercial use of the sun's rays for such purposes is one of economics.
A major factor contributing to the relatively high cost of solar energy converters is the cost associated with the manufacture of the collector component; i.e., the reflector or lens which acts to collect and focus solar rays to form a region of intense heat. To produce significant quantities of heat or electrical energy economically from solar energy, collectors of substantial size are required. To satisfy this requirement, large trough-shaped reflectors have been proposed and, in some instances, are being utilized as the collector element. While such reflectors are substantially less expensive than refractive elements of comparable light-gathering power, their manufacturing costs are, for the size required, still at a level which renders large-scale energy production economically marginal. To date, trough reflectors have been conceived and/or made of glass, space-frame structures and segmented sheets, or molded composite plastic materials. Such conventional reflectors are not only difficult to fabricate, but also they require the use of relatively expensive materials.
The trough collector must possess a high-accuracy reflector surface and maintain the surface profile and its structural integrity throughout environmental extremes. The most strenuous structural test is from wind-induced loads. The bending load is proportional to the collector width, and the torsional load is proportional to the square of the collector width.
To date, the most cost-effective trough collectors have been of two types. First, there is the torque-tube design that incorporates a tube or pipe that forms a backbone for the trough collector and absorbs the wind-induced bending and torsional loads. An example of this design is disclosed in U.S. Pat. No. 4,135,493. This design uses a number of thin transverse ribs attached to a pipe backbone. The reflector sheet is loaded by end pressure against the parabolic shape cut into the transverse ribs to generate the parabolic trough shape. A second sheet of material is required on the back surface of the ribs for strength and to keep wind pressure from lifting the reflector sheet off the front surface of the ribs. The bending and torsional strength are a simple strength of materials function of the properties of the torque tube. The widest collector built to date by this method is seven feet and this appears to be the design limit when balanced against the cost of stronger materials.
Second, there is the monocoque design that uses a stressed reflector skin and a stressed rear skin that are separated and supported by ribs. An example of this design is disclosed in U.S. Pat. No. 4,240,406. In this design the structure gets its strength from the attachment of the front reflector sheet and a rear sheet to fairly thick transverse ribs, similar to the design of modern airplane wings. This design requires a fairly thick transverse rib so that the reflector sheet and the rear sheet may be fastened to it with screws. The widest collector built to date is eight feet which appears to be the design limit.
Both of the above-described designs are conceived with the initial constraint that no structure may obscure the entrance aperture since this will lower performance. Typically, this initial constraint has been widespread in the solar concentrator field and may explain, at least in part, why the present invention has not been previously conceived.
Some collectors such as that disclosed in U.S. Pat. No. 4,205,659 have used a clear protective covering of plastic film which may make a small contribution to the torsional stiffness but this is severely limited by: (1) the small effective cross-sectional stress area since the recommended material is only 0.004 inch thick; (2) the requirement for a long-life rigid edge fastening; and (3) the requirement of permanent pre-stressing of the material since it is non-rigid. Further, the recommended material transmits 90% of the incident solar energy, which means the entire collection system pays a 10% efficiency loss penalty which cannot be justified by this method of structural stiffness contribution.